First Light, otherwise known as the start of Nautical twighlight, is when the sun is approximately 12 degrees below
the horizon. The start and finish of Nautical twilight marks the practical limits of unassisted human vision on a moonless night.

Dawn is the start of Civil twilight, when the sun is approximately 6 degrees below the horizon.

Civil twilight is generally considered the limit of useable daylight; before
dawn or after dusk artificial light is needed for most common outdoor activities.

Sunrise is when the leading edge of the sun crosses the horizon as it rises.

Transit is the time at which the sun is at its highest point in the sky.

Sunset is when the trailing edge of the sun crosses the horizon as it sets.

Dusk is the end of Civil twilight, when the sun is approximately 6 degrees below the horizon. By dusk,
most street lights will have come on and it will start to "feel" dark – there will still be a lot of
light in the sky, but the perception is that it's the beginning of the night rather than the
end of the day.

Nightfall is the end of Nautical twilight, when when the sun is approximately 12 degrees below
the horizon. By the end of nautical twilight, the horizon will be indistinct
and unlit ships will no longer be silhoutted against the sky.